1. Field of the Invention
This invention relates generally to a traction control system and, more particularly, to a traction control system using fuzzy logic to control wheel slip of the drive wheels of a train locomotive in order to maximize the coefficient of friction between the drive wheels and the rail interface.
2. Discussion of the Related Art
As is well understood, train locomotives, such as diesel electric locomotives, used to move railway cars along a dual rail configuration are propelled by exerting torque to drive wheels associated with the locomotive that are in contact with the rails. The power to propel the locomotive is first developed as mechanical energy by a high horsepower diesel engine. The diesel engine drives a generator that converts the mechanical energy to electrical energy. The electrical energy is then converted back to mechanical energy by a series of traction motors. One traction motor is rigidly connected to each axle associated with a pair of drive wheels such that when the axle is rotated, the drive wheels also rotate. In this manner, each traction motor is in parallel with the generator and rotates independently of the other axles. The specifics of the mechanisms for propelling a locomotive are well known.
Friction between the drive wheel and the rail interface provides the traction for causing the movement of the locomotive. If there were no friction between the drive wheels and the rail surface, the wheels would rotate and slip relative to the rails without providing movement to the locomotive. The friction force is a function of the coefficient of friction between the drive wheels and the rail interface, and the downward force exerted by the locomotive on the rail. Wheel slip is the value that characterizes the difference in linear speed between the locomotive and the drive wheels. The more friction between the drive wheels and the rail surface, the more traction can be generated, and thus, the more propelling force is available to move the locomotive. It is possible to increase the friction force between the drive wheels and the rail surface by increasing the weight of the locomotive. However, certain well understood engineering and economic factors make this option unattractive. Therefore, locomotives incorporate traction control systems in order to maximize the friction force between the drive wheels and the rail surface, so as to maximize the propelling force of the traction motors.
A traction control system may have two control variables that can be used to control the amount of wheel slip of the drive wheels relative to the rails. First, the power output of the generator can be reduced, which leads to lower traction motor torques, which in turn leads to less force causing the drive wheels to spin. However, reduced generator power provides less power to propel the locomotive. Second, the locomotive can drop sand in front of the wheels, thus creating a greater friction relationship between the drive wheels and the rail surface. However, limited payload and cost of sand require that this method of control be used conservatively. Consequently, the only practical way to maximize the friction force is to maximize the value of the coefficient of friction between the drive wheels and the rails.
When torque is applied to the drive wheels during operation of the locomotive, the wheels will begin to slip at an increasing rate until the slip reaches an equilibrium wheel slip level when the torque acting on the wheels balances out. More or less torque will cause the wheels to slip at a different rate, thus changing the time at which equilibrium is reached. If the friction/slip relationship between the drive wheels and the rail interface increased monotonically, no control system would be required. Under most rail conditions, however, the graphical relationship between the coefficient of friction and the wheel slip reaches a peak at a location depending on the torque being applied and other system conditions, and then declines.
FIG. 1 shows a graphical relationship between the coefficient of friction on the vertical axis and the percent of wheel slip on the horizontal axis for dry, wet and oily rail conditions. A graph line for each coefficient of friction versus percentage of wheel slip is given for the different rail conditions in each direction of rotation of the drive wheels. As is apparent by viewing FIG. 1, under dry rail conditions, the coefficient of friction is at its maximum at about 10% wheel slip. It is noted that under ideal dry rail conditions, the torque that a locomotive engine can generate is not high enough to cause the wheel slip to increase to this value. Once the rail becomes wet and/or oily, the percentage of wheel slip at which the maximum coefficient of friction is attained increases. Particularly, for a wet rail condition, the maximum coefficient of friction is at approximately 15% wheel slip. For oily rail conditions, the maximum coefficient of friction is given at about 30% wheel slip. Wheel slip above 25%, however, can cause excessive wear or even damage to both the wheels and the rail. The region of each graph line before reaching the peak is a stable region of operation, however, the region of each graph line beyond the peak is an unstable region in which predictable wheel rotation is indeterminable.
The goal, therefore, of a traction control system that attempts to maximize the coefficient of friction is to operate as close to the peak of the friction/slip curve as possible, while avoiding the unstable region. The general strategy of the traction control system is to estimate the slip level at which the friction/slip curve reaches its peak value, and to regulate the system so that the wheel slip does not exceed this value. When the wheel slip is close to the unstable region, control action must be taken to remain in the stable region. This task is made more difficult because the exact characteristics of the friction/slip curve is never known as rail conditions can rapidly change causing the estimated peak to be at a higher slip value than the actual peak. Abnormal rail conditions may cause a friction/slip characteristic that increases asymptotically to a maximum coefficient of friction. A traction control system must limit wheel oscillation while correcting the estimation of the friction/slip curve peak.
Prior art traction control systems generally consist of cascaded single input and single output controllers. The innermost controller loop controls the generator's field current by varying the generator's field voltage source. A reference field current is generated by a generator output controller for establishing the generator field current. The generator output controller regulates the generator so the generator's operating characteristics follow a desired pattern based on a system model of the locomotive. The system model would include a generator model, a generator field model, and a model representing the field current controller and the generator output controller. A simplification can be made because the closed loop response of the actual generator controller is known.
Traction control system models which define the rotation of an axle generally assume that the wheels associated with an axle rotate at the same angular velocity and experience the same coefficient of friction. In reality, differences in the coefficient of friction exist between the two wheels of an axle exists such that torsional forces are generated in the wheel axle and differing angular velocities occur for the two wheels.
Because of the many factors which effect the friction/slip relationship, it is necessary to incorporate traction control models that are robust enough to compensate for these variances so as to increase the coefficient of friction between the drive wheels and the rail interface. It is therefore an object of the present invention to provide a traction control system with increased capabilities over the prior art systems.